1. Field of the Invention
Aspects of the present invention relate to a cathode active material and a cathode and a lithium battery using the same, and more particularly, to a cathode active material doped with a metal, and a cathode and a lithium battery including the same.
2. Description of the Related Art
As a cathode active material for lithium batteries, transition metal compounds, such as LiNiO2, LiCoO2, LiMn2O4, LiFePO4, LiNixCo1−xO2(0≦x≦1), and LiNi1−x−yCoxMnyO2(0≦x≦0.5, 0≦y≦0.5), or lithium oxide, are used. A representative cathode active material is LiCoO2, but LiCoO2 is relatively expensive and has a limited actual electrical capacity of 140 mAh/g. Moreover, if 50% or more of lithium is removed from LiCoO2 by charging, LiCoO2 exist in the form of Li1−xCoO2 (x>0.5). The oxide in the form of Li1−xCoO2 (x>0.5) is unstable and unsafe.
Specifically, only about 50% of its theoretical capacity of LiCoO2 is used, due to its structural instability. Only about 140 mAh/g can be utilized when 0.5 or more of Li remain in Li1−xCoO2 (x<0.5) and when the charge voltage is 4.2V versus Li metal. If, in order to utilize the LiCoO2 to 50% or more of its theoretical capacity, the charged voltage is increased to 4.2V or more, LiCoO2 becomes Li1−xCoO2 (x>0.5), and the crystal structure of the Li1−xCoO2 (x>0.5) undergoes phase transition from a hexagonal phase to a monoclinic phase, thereby becoming structurally unstable, and as the cycle progresses, the electrical capacity is drastically reduced. In order to solve these drawbacks, cathode active materials such as LiNixCo1−xO2 (x=1, 2) or LiNi1−x−yCoxMnyO2(0≦x≦0.5, 0≦y≦0.5) have been used, but such cathode active materials do not show good swelling resistance under high temperature. Lithium metal oxides in the form of Li[LixM1−x]O2 (x>0, M is a plurality of transition metals) provide an increased electrical capacity of 250-280 mAh/g. However, the lithium metal oxides, including excess lithium, have low electrical conductivity, poor cycle properties, and their high rate properties are deteriorated.